温州鳌江近海建设工程环境影响潮汐潮流数值模拟

温州鳌江近海建设工程环境影响潮汐潮流数值模拟
吕和娜;汪一航;张钊;单慧洁;岳云飞
【摘要】This paper have col ected the measured tides and certain tidal current data in different stages of many projects during past three to five years near the Aojiang River. The harmonic method is used to analyze tide and tidal current data observed at five stations in the sea adjacent to Aojiang River. The results show that the tide is mainly regular and semidiurnal in the sea near Aojiang of Wenzhou. The tidal amplitudes of
M2 constituent are between 170 cm-193 cm and the lags are between 260°-280°. According to the comparison of analytical results of harmonic constants of these stations in 2007, 2010 and 2011, the maximum change of tidal amplitudes and phase-lag range for the main semidiurnal tides (M2, S2, N2), the diurnal tide (K1, O1) and the shal ow water tide (M4, MS4, M6) are 1.8 cm-4.4 cm and 3°-7°, respectively. After analyzing the tide and tidal current characteristics of Aojiang River, this paper uses an unstructured grid and Finite-Volume Coastal Ocean Model (FVCOM) to test the results of simulation. The simulated results agree well with the measured data. The new shoreline and depth which are produced by the construction projects closed in important major years, and the tide and tidal current field for the new shoreline and depth are constructed, which describe the superimposed influences of construction engineering in Aojiang estuary.%收集了近年来鳌江口附近海域多个工程不同阶段5个潮位站的3～5年潮位实测数据和部分海流实测资料,通过对鳌江口附近海域的不同年份的水
位资料进行潮汐调和常数分析,鳌江近海海域主要为半日潮区,其中M2分潮的振幅在170 cm ～193 cm；迟角在260°～280°之间,这些站的2007年、2010年、2011年调和常数分析结果相比,主要的半日分潮M2、S2、N2，全日分潮K1、
O1及浅水分潮M4、MS4、及M6等分潮振幅、迟角的最大变化分别在1.8
cm ～4.4 cm和3°～7°之间。

在初步掌握了鳌江口潮汐潮流特征的基础上，采用无结构的三角形网格和有限体积法的FVCOM海洋数值模型,进行模拟结果验证,计算结果与实测数据符合良好。

构建重点年份建设工程合拢产生新的岸线水深的潮汐潮流场，刻画鳌江口建设工程的叠加影响。

【期刊名称】《海洋通报（英文版）》
【年(卷),期】2014(000)002
【总页数】15页(P13-27)
【关键词】鳌江口附近海域;潮汐;潮流;环境影响;数值模拟;后评价
【作者】吕和娜;汪一航;张钊;单慧洁;岳云飞
【作者单位】宁波大学理学院，浙江宁波315211;宁波大学理学院，浙江宁波315211; 宁波非线性海洋与大气灾害系统协同创新中心，浙江宁波315211;国家海洋局温州海洋环境监测中心，浙江温州 325027;宁波大学理学院，浙江宁波315211;宁波大学理学院，浙江宁波315211
【正文语种】中文
The sea is the advantage and potential for the economic development in Wenzhou. The reclamation plays a positive and important role in economic growth of Wenzhou. Moreover, it is one of the critical ways to relieve the
shortage of land and spatial resources.According to “The development planning of the coastal industrial belts between Wenzhou and Taizhou”, there will be 34 terms of tideland reclamations, possessing about 370
km2areas, along the coast of Wenzhou by 2020 and among which the Aojiang is a typical one. The large and relatively fast-developed reclamation provides the industry improvements of the coast with developing space. However, it also increases the tension on the coast of Wenzhou resulting in many negative benefits for the reclamation, which can usually lower water exchange ability and water self-purification ability, leading to sedimentation and increscence of pollution contents in water. These problems will not only cause a lot of the marine environmental and ecological problems and so on, but also restrict the sustainable development of the marine economy in Wenzhou. The study of reclamation project in Wenzhou on marine environmental impact shows that a single reclamation has little impact on the marine environment in Wenzhou, and the research on the effects of the cumulative reclamation on marine environments is still in the primary stage due to some factors, such as the lack of the typical reclamation engineering group and valuable marine monitoring data, etc. Therefore, the study about effects of the reclamation on marine environments in Wenzhou is a practical and cutting-edge issue. Tides and tidal currents are the typical features of sea that can reflect the marine and ecological environment. Hence, several reclamation projects as well as their effects on tide and tidal currents need to be paid attention for providing some more valuable instances for
marine environment.
Many Chinese and foreign researchers have studied the effects of the reclamation on tide and tidal superposition. Until recently, there are already some related researches on effects of offshore construction and engineering in Wenzhou on tide and tidal currents, such as setting up widespread mathematical models of two-dimensional tidal currents and suspending sediment transport including Oujiang River, Feiyunjiang River and Aojiang River. Those projects focus on the long-term reclamation projects’ effects on the distribution of the suspended sediments, profiling sand transport rate as well as the evolution of seabed scour and deposition, which belongs to three main rivers flowing into the sea and coastal waters (Wang et al., 2008). Shen et al. (2009) built 2D tidal current numerical model and water exchange model for Wenzhou offshore and estuary, which simulated the effects of the reclamation in Wenzhou on the water exchange capacity for Oujiang River, Feiyunjiang River and Aojiang River estuaries. These studies focused on a large scale area, but didn’t accurately assess the changes of the marine dynamic environment of the offshore area. At the same time, some domestic scholars have studied the effects of the cumulative reclamation on tides and tidal currents, such as the analysis of the characteristics of the dynamics and concentration and erosion and deposition in the Bacao Bay(Jiang et al., 2005). Using the Delft3d to model the hydrodynamics of Aojiang estuary, the reclamation influence has been analyzed from tidal prism, tidal level, velocityand residual flow(Wang et al., 2013); they also retrospectively evaluated the
effect of Shenzhen land reclamation engineering on the tide, tidal volume, the marine environment and so on (Guo et al., 2005); the topography and coast line of 1963, 2003 and 2010 were simulated based on POM, and the impact of reclamation on hydrodynamic cumulative effect of the Xiangshan Bay surrounded has been studied by Zeng et al.(2011); Lingni embankment apart of Wenzhou Peninsula Engineering is the first marine engineering implementation of post project analysis on the marine environmental impacts in China (Wang et al.,2010); Wang et al.(2000) and Wang et al.(2010) utilized the establishment of other mathematical models to study the effects of reclamation engineering on the marine environment as well. These scholars focused on the study on river runoff of Aojiang or any other area, while, in this paper we have collected the tide and tidal current data from Aojiang estuary, and then compared and simulated superposition effects and influences of working condition of each process of Aojiang estuary by using the observed data correlation method and combining with irregular triangular grids and finite volume method of FVCOM model. Then the superposition effect of reclamation activities on the Wenzhou Aojiang offshore movement and the scope and degree of impact on the surrounding seas were revealed by using all the information and data, and it is expected to provide examples for the study of the effects of the cumulative reclamation on marine environments, and provide a reference basis for formulating reclamation of superposition effect measures.
We have collected three typical data of tides and tidal currents from the
observation of five stations’ during the reclamation engineering from 2007 to 2013, which locate in Aojiang offshore. On the basis of analyzing the characteristics of tides and tidal currents of Aojiang, we have used the FVCOM to simulate local tides and tidal currents of Aojiang in order to further verify our calculated results. Then the effects of the reclamation on tides and tidal superposition for each typical reclamation closed time were analyzed by comparing the measured data and simulated data.
Aojiang estuary and offshore is research area in this article, and the exact coordinates are 120.52°E - 120.89°E, 27.42°N - 27.78°N (Fig. 1). As for this area, east-west width is about 36 km, and north-south length is about 40 km, whereas the offshore coastline is very complex. Aojiang is about 90 km long and 1 km wide at the mouth, which is termed as a typical horn. Its climate type is subtropical maritime monsoon, which has mild and humid climate, four distinct seasons and abundant precipitation. The annual precipitation peak is between March and September, among which, March and April are the spring period, May and June are the Meiyu period, July and September are the tropicalstorm rain period, and the annal precipitation is diminishing from the land to the island(Li et al., 2012). There are Si Yu Island, San Yu Island, Shang Touyu Island, Suan Yu Island, Tie Tingyu Island and other islands outside Aojiang estuary, and Man Toushan Reef and Feng Huangdan Reef locating in the north of Aojiang estuary. The Aojiang estuary and coast silt year by year due to the upstream flows with sediments and it is affected by sea continental shelf sediment movement sending sediments elsewhere, and expanding the
deposition surface, which lead to form the broad estuary and coastal area of tidal flat (Yao et al., 1998; Can et al., 2013；Nanjing Hydraulic Research Institute, 2005). From 1064(the first year of Northern Song Dynasty - Zhiping) to 1979, the coast had moved 2 - 7 km to East China Sea, and the average annual rate was 2.2 m - 7.7 m, while, the rate was 10 - 20 m after the founding of the People’s Republic of China. In the south of Aojiang, there are 17 000 acres of reclamation of Jiangnan, which belongs to Cangnan county; in the north, Xiaonan belongs to Pingyang county which has the reclamation of 2 000 acres, and the developed tidal flat area is very broad. The north area connects with Feiyunjiang, and the south area to Pi Pa Hill, with the total reclamation of 255 000 acres. Among them, 120 000 acres have been out of the water; 50 000 acres can be reclaimed in the Jiangnan region; and there are 135 000 acres above the theoretical datum. With the rapid development of Wenzhou economy and the growth of population, there are some reclamation project (Fig. 1) built or to be built for the shortage of land resource and the sustainable development of Wenzhou. The reclamation of Xiwannan, which locates in the north of Aojiang estuary and was constructed from 1999 to 2002, has the reclamation area of 3.48 km2(5 220 acres) and dike of 5 116 m. The polder of Jiangnan, which locates in the east coast of Cangnan at the south of Aojiang estuary and was constructed from 2007 to 2010, has the reclamation area of 4.69 km2(about 7 031 acres) and the total area of the ocean is 29.57 km2. The reclamation of Huarun Cangnan power(including the breakwater), which was constructed from 2008 to 2011, is located in
the north of Jiangnan polder.
2.1 Analysis of the tide features
In this paper, we compare the harmonic constants (Tab. 1) of five stations (Fig. 2) in different years, which locate in Aojiang coastal waters. Through the tide station harmonic analysis results, and according to the formulawe calculate the tidal coefficient of each tide station and find that the values are between 0.21 and 0.35, and consequently this area has semidiurnal tides, and the ratios ofshow us that thevalues are 0.007 and 0.01 in Beiji Island and Nanji Island, respectively, and increasing to 0.13 in Aojiang station. The phenomenon shows the tide from the regular semidiurnal tide of Beiji Island to the informal semidiurnal shallow sea tide of Aojiang mouth(Fang et al., 1986). Based on the analysis of the observation data, the tide reaches more than 4 m in the study field, and the tidal day phenomenon is significant, which indicate that the obvious characteristics of semidiurnal tide; the duration of rise and ebb is consistent in the Aojiang estuary, while the spring duration is reduced and ebb duration is increased when it enters the mouth, so the spring duration is less than that of ebb.
Tab. 1 shows the harmonic constants of five tide stations in the sea adjacent to Aojiang River, and the M2constituent dominates in Aojiang district. Considering the results of the harmonic constants of main constituents from the five stations of different years, the majority of partial tide amplitudes have fallen slightly, but the decline range is very limited. The changes of partial tide amplitude are within 5 cm in the Aojiang
surrounding waters, and the biggest difference of phase lag is 8.3°, and the range is between 1.5% and 3.0%, which means the range ability is small. Meanwhile, the biggest changes between adjacent years of M2amplitude
in Aojiang, Shipeng and Beiji stations are 1.6 cm, 2.7 cm and 2.7 cm respectively, and the maximum ratios are 0.85%, 1.39% and 2.46% respectively, and the variational maximum value of phase lags are 5.4 °, 2.3° and 5.4° respectively, so that the effect of M2constituent is not significant. We also can know the other partial tides, K1, O1, S2and N2are similar to
M2, and the range ability of amplitude and phase lag are very tiny. And the obvious areas for the shallow water tide M4, MS4are in Aojiang and Shipeng stations, not in Tong Panshan and Beiji stations, and the biggest difference of amplitudereaches 5 cm, while the changes of them are relatively stable, and M6constituent feature is not obvious in all stations. Our computed results show that the cumulative effect of all reclamation on tide is inconspicuous, and the change of tide characteristics is not significant.
2.2 Analysis of the tidal current feature
As shown in Fig. 2, five consecutive tidal stations, A01-A05 near Aojiang in Wenzhou, are selected and analyzed as typical stations. The property in this sea area is the regular semidiurnal tidal current. The flow velocity decreases gradually from the surface to the bottom, divided into small change in the flow direction, and the flow exhibits reversing current characteristics. In the surrounding of Aojiang, the direction of the spring tide is mainly NW direction, and the ebb direction is mainly SE. The
direction for the reversing current is parallel to the coast, while individual site has the rotary current in Cangnan. The spring velocity of Aojiang mouth is mainly influenced by the runoff in the upper, so that the runoff is small in the dry season and spring tide current velocity is sufficiently larger than that of the ebb. The runoff gets bigger in flood seasons, which causes the decrease of spring tidal current velocity, raises duration and increases the ebb velocity and the ebb tide duration, so that the ratio of rise and ebb velocity decreases.
According to the estuary of Aojiang to the outlets, from the velocity and duration of spring and ebb tidal current, the ebb velocity and duration are greater than those of flood tide due to the influence of the upstream runoff, and they are both gradually weakened;and the estuary belongs to a typical strong tidal creek estuary, which is influenced by the runoff and tide; and its surrounding waters belong to typical rectilinear currents, and spinning is not strong. Since the Aojiang river mouth area has regular semidiurnal tidal currents and the M2constituent is as main constituent, we analyze the ellipticity value K and the long axis of the ellipse of M2of five different years and continuous flow stations. We find that the ellipticity value K of M2in 2011 has changed insignificantly by comparing with those in 2010 and 2007, and the values are between 0.01 and 0.03 in all the measured stations. At the same time, the value of the long axis of M2in 2011 is slightly less than that in 2007, with the average velocity of 0.05 m/s, and the direction represents the tidal wave propagation direction, and its size represents the maximum velocity of tide of the corresponding tidal
constituent. Therefore, superimposed effect of the outer reclamation engineering on the surrounding sea tidal movement is not significant in Aojiang surrounding waters.
3.1 Establishment and validation of the model
3.1.1 Introduction for FVCOM
Due to the limitation of the measured data in time and space, it is difficult to systematically evaluate data in future only with the measured data. However, the numerical model based on a large amount of measured data can more accurately simulate in the whole area and can verify the related numerical data. It is an efficient evaluation method for post evaluation. We have built three dimensional hydrodynamic models FVCOM (an unstructured grid, Finite-Volume Coastal Ocean Model) (Chen et al., 2006) by analyzing the terrain and land boundary data of 2007, 2010 and 2011 in this study, which investigate the simulation of the superposition effect and influence of each process in Aojiang waters. The FVCOM employs the finite-volume method which can well combine the advantage of finite element method and the finite difference method. It can guarantee the conservation of mass, momentum and energy for the computation though th e coast is complex. It uses a σ-coordinate in the vertical direction that can be good to adapt to the changes of terrain, and can use unstructured triangular mesh nonoverlapping in the horizontal direction, which can fit the complicated coastal and can make refinement mess for the focus area. It uses the Mellor-Yamada 2.5 turbulence closure model for calculating vertical mixing coefficients, and uses the Smagorinsky formulation for
calculating horizontal mixing coefficients. In addition, it applies a mode-splitting technique for splitting the fast moving gravity wave modes outside from the slow moving internal. Therefore, we have built FVCOM models forthe coast of Aojiang waters, which are complex, and have harbour large tidal flat area and uneven distribution in the area. The level, velocity, and flow are validated using the data of 2007, 2010 and 2011. 3.1.2 Setting up model
As shown in Fig. 3, this research mainly investigates Aojiang area, for which the north-south length is 155 km and east-west width is 134 km, and the horizontal resolution varies from 50 m in the offshore to 2 000 m in the sea, and there are six distributed layers in the vertical direction. Though there is debugging in this mode, the computational time steps of 6s and 30 s are used for the external and model respectively. There are 168 307 units and 88 363 nodes in the whole area. This design of the mode grid can depict the complex of coastline, which is caused by the ocean engineering construction.
The friction coefficient of bottom is as follow:
where Z0is the roughness of the seabed and the value is 0.01 in this simulation after debugging, and k is the Karman constant and the value is 0.4, and Zabis the distance between the latest grid and the bottom of the sea. In this mode, the value of bottom friction coefficient is 0.004.
The open boundary conditions are specified in the form of sea surface elevation:
where ζ is the tidal level, ranging from 1 to 8, and just represents the eight
main constituents, including M2, S2, K1, O1, N2, K2, M4, MS4. ωiis the tidal angular velocity. ƒiand μiare the nodal factors and nodal phase correction respectively. Hiand giare the tidal harmonic and constants, and their values are based on the tidal harmonic and constants which we have collected in Wenzhou offshore and the previous results taken as the reference after debugging(Chen et al., 2003). The model starts from static sea and continues to compute for one month after obtaining stable tide, employing the least square method to analyze the harmonic analysis and features of the tide and tidal currents.
3.1.3 The validation of tide elevation
In order to further verify our calculated results, we compare time series in one month for two tide stations: Aojiang and Beiji (Fig. 2). Actual observed sea levels and stimulated ones are shown in Fig. 4. The mean-root-square error for Aojiang and Beiji are 0.18 m (2007.04) and 0.19 m (2011.04), 0.11 m (2007.07) and 0.15 m (2007.07) respectively. The simulated tide elevation is more accurate. The results show that the stimulated model has better agreement with the actual tidal change of Aojiang in Wenzhou.
3.1.4 The validation of tidal current
Comparison of two tidal stations A02 and A05 between observed and modeled values of flow velocity and direction in the surface and underlying are shown in Fig. 5. Both of the tidal currents in two sites are regular semidiurnal tidal currents, which can also be concluded from the simulation results. From the contrast of flow velocity and direction, the simulation results are basically consistent with the measured ones, and the
direction is better in agreement with the actual measurement data. The mean-root-square errors of flow velocities in A02 and A05 in the surface and underlying are 9.8 cm/s, 8.9 cm/s, 7.8 cm/sand 5.7 cm/s，respectively. The mean-root-square errors of flow direction are 17.3°, 22.4°, 19.1° and 21.2° respectively. The simulated flow velocity and direction fit well with the observation. The results show that the mode can well response the change of tidal currents in Aojiang.
3.2 The change of the flow field
From the spring and ebb tide graphs of flow field in different years, it shows that the reclamation project does not change superimposed effect in the following state in the sea area near Aojiang and the overall
fra mework of tidal current field in Aojiang doesn’t changeneither. The flow direction is mainly from the northwest to southeast, which is caused by the morphology of terrain. The water mainly flows from southwest to northeast during the spring tide, and from northeast to southwest during the ebb tide. The graphs of flow fields for spring and ebb tides in 2007and 2011 are shown in Fig. 6 and Fig. 7, respectively.
As shown in Fig. 6, the overall flow field has little change before and after the reclamation of Jiangnan polder and Cangnan Power when ebb happens in Aojiang estuary. There is little change in the edge of the reclamation for the tidal current in the size of the current, but the direction largely follows along the beach to the southeast direction. Fig. 7 indicates that the tidal currents south of the Cangnan power are consistent before and afterthe works. Although the direction diverts to the north based on
the northwest direction reclamation before the Jiangnan polder, and the current velocity increases slightly, other sea areas have little change. Above all, the flow field has no significant change in Aojiang before and after the reclamation works.
Through the analysis and calculation for the real measured data for Aojiang offshore superposed influences of tide and tidal currents from the reclamation can be concluded as follows:
(1) Through comparing and analyzing the measured data of 2007, 2010 and 2011,the tide level has little change in these years. The difference of tide amplitudes and lags for M2, S2,N2, K1, O1, M4, MS4and M6are in 1.8 cm - 4.4 cm and 3º - 7º respectively. These data demonstrate that cumulative effects of all reclamation works on tides are inconspicuous. (2) By the comparison with the realistic data, the result shows that ellipticity value K of tidal component M2in 2011 had no obvious change, compared with those in 2010 and 2007. The value of the long axis of M2in 2011 is slightly shorter than that in 2007. Therefore, the reclamation works slightly change the superposed influence of the tide and tidal currents surrounding Aojiang.
(3) Both the stimulation and actual measurement confirm the accuracy of the model. The good agreement of stimulation results with the real data suggests this research reasonable. Through the analysis of the flow field of flood and ebb in different years, it can be found that the change is not significant in Aojiang.
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